Method of regenerating aluminum anodizing solution



Feb. 20, 1951 0, BORNGESSER 2,542,112

METHOD OF REGENERATING ALUMINUM ANODIZING SOLUTION Filed April 16, 1945 FIG.

OSCAR R. BORNGESSER A TTOR/VEVS.

Patented Feb. 20, 1951 METHOD OF REGENERATIN G ALUMINUM ANODIZING SOLUTION Oscar R. Borngesser, Wichita, Kans., assignor t Boeing Airplane Company, Wichita Division, Wichita, Kans., a corporation of Delaware Application April 16, 1945, Serial No. 588,533 g It has been customary for some time'to coat articles of aluminum and aluminum alloys with an oxide coating for the purpose of preventing later corrosion. Such oxide coating has been produced by an electrolytic anodizing process, in which the articles being treated form the anode, while various acids, such as chromic acid, sulfuric acid and oxalic acid, have been utilized for the electrolyte.

Chromic acid is used quite commonly in such electrolytic anodizing treatment. The usual practice, however, to regulate the acid content of the electrolytic bath for anodizing effectively aluminum articles requires considerable replacement of chromic acid. Such periodic replacement not only is expensive but has effected variations in characteristics of the electrolyte sufficient to impair the uniformity of the oxide coating produced.

It is the principal object of my invention to devise a technique for regenerating the chromic acid, or other acid, of -anelectrolytic aluminum anodizing bath, and to provide suitable equipment for carrying out this technique.

It is a further object to enable such regenerating operation to take place continuously, so that the characteristics, including the acidity of the anodizing liquid, can be maintained substantially constant, enabling the most effective anodic coating to be obtained with the highest efiiciency.

Still another object is to construct the apparatus for carrying out my process so that it may be operated continuously with the minimumof attention.

Other advantages of my process and apparatus, and the manner in which the aforementioned objects may be achieved, are discussed in detail in the following description of a .representative type of installation and its mode of operation.

Figure 1 is a diagrammatic vertical sectional view of typical apparatus capable of performing my method, and Figure 2 is a plan view of one unit of such apparatus.

In a customary anodizing process a number of factors may be varied over a considerable range, which variation has a greater or lesser effect upon the adulteration of the electrolytic bath, and consequently on the quality of the anodic coating on the aluminum articles being treated. Reference to aluminum articles is intended to include alloys of aluminum containing other metals, but in which aluminum predominates. Such variations in the anodizing process proper are pertinent to my invention only in the ability of my process to eifect the required regeneration of the anodizing solution. On the contrary, whatever characteristics of the anodizing solution may be established, my regeneration process is very helpful in maintaining constant such desired charac- 5 Claims. (01. 204 5s) teristics over a prolonged period, in fact indefinitely.

The most important and critical factor of the anodizing process is the maintenance of proper current density. Customary practice attempts to hold the current density constant at a selected value between 1% and 2 /2 amperes per square foot of surface area of the aluminum articles being anodized. This item is of paramount importance because the thickness and weight of the oxide film produced, other factors being the same, is directly proportional to the current density. It has been found, however, that on application to the bath of a constant voltage the current density decreases as the pH of the solution increases. To preserve a constant current density, therefore, the pH of the anodizing bath must be static within a very small range of fluctuation, such as 0.1, if the voltage impressed does not change. For best results it has been found that the pH should be within the range of 0.5 to 1.0, and I prefer that it be kept at a definite value Within the range of 0.6 to 0.9. 1

In preparing the electrolyte solution initially the desired pH may, of course, be readily established precisely. Chromic anhydride, CrOa, may be dissolved in water according to the following equation until the desired concentration and pH have been achieved:

When electric current is passed through this anodizing solution electrolysis of the electrolyte progresses as is usual in the case of an electrolyte formed by an acid having a nonvolatile radical. Thus hydrogen is liberated as a gas at the cathode, and by reason of a secondary electrode reaction the chromate radical at the anode decomposes the water to liberate oxygen. All of such oxygen'does not effervesce, however, but instead a portion of it reacts with the aluminum of the anode to form theanodic oxide coating. The following equation represents this reaction:

H.0r.01+A120.+(s+w H2+(;)0. T (2) The aluminum oxide thus formed remains tightly adherent to the anode.

This is not the only chemical combining action, however, for almost an equal quantity of aluminum reacts directly with chromate radical to form aluminum dichromate according to the following equation:

This dichromate enters into solution. With the electrolysis, therefore, at least two secondary chemical reactions, represented by Equations 2 and 3, are progressing simultaneously during the anodizing process. It will be noted that neither the direct electrolysis of the acid solution nor the anodizing action represented by Equation 2 consumes any chromic acid. The formation of the aluminum dichromate by the reaction of Equation 3 does, however, remove aprogressively increasing quantity of the acid radical from active participation in the anodizing action.

As the formation of aluminum dichromate progresses the pH of the electrolyte increases. It is known that the rate of both reactions 2 and 3 decrease as the pH rises, and the proportion of the aluminum reacting according to Equation 3 tends to increase somewhat. In order to keep the anodizing conditions reasonably uniform by maintenance of approximately constant pH' it has heretofore been the practice periodically to draw off a portion of the electrolyte and to add chromic anhydride to adjust the pH to its initial value. Despite such manipulation, however, the concentration of aluminum dichromate continued to increase until the anodizing action became so inefficient that it was no longer practical to reduce the pH of the electrolyte to its optimum value by that expedient. When the pH rose to 0.9 it was customary to discard the entire solution and to replace it with fresh-chromic acid solution.

It will be appreciated, therefore, that if the dichromate of aluminum and other metals of the anode and of impurities in the water could be removed continually from the electrolyte and replaced by an equivalent amount of chromic acid, the electrolyte would notbecome adulterated. In such case the pH, and consequently the current density, would. remain substantially constant, to promote uniformity of the anodizing action. Moreover, if the chromate radical could be separated from the metals of the dichromates and returned in aqueous solution to the anodizing tank it would be unnecessary to add any chromic anhydride, because, as pointed out above, none of this ingredient is expended either in the electrolysis or in the anodizing phase of the reaction expressed by Equation 2. Both of these results are accomplished by the regeneration process of my invention.

In carrying out my process I remove electrolytic liquidfrom the anodizing tank proper for regeneration treatment. While it is preferred that the regeneration process be carried out continuously, it may be operated intermittently, even though the anodizing process itself is continu ous, For continuous regeneration anodic tank electrolyte is withdrawn at a predet'ermi ned rate. treated to remove its metallic content, principally aluminum, from it, and when thus regenerated is returned to theanodizing tank. All'the'met'al lic content may not be removed by a single treatment of my process, but enough of it is extracted so the quantity of metal dissolved in the anodic electrolyte is not excessive for good anodizing and remains practically constant after it has reached an equilibrium condition, where the regeneration treatment is continuous. Some chromic acid radical may be lost in the regeneration process, but as compared to the chromic acid rendered in-' efiective without my treatment such loss is negli ible.

My process contemplates electrolyzing the liquid withdrawn from the anodizing tank in a i cell divided by a porous partition. The anode and the cathode of such regeneration cell, which may be of lead, copper, or other material not reactive with the solution to be regenerated, are

located on opposite sides of the partition to seg regate the anolyte from the catholyte. The porosity of the cup should be sufficient to enable gradual seepage of the liquid through it and free migration of the ions between the anode and cathode. Examples of materials having suitable porosity for this purpose are porcelain and Alundum. After passing current through the solution in such a cell for a time the pH of the catholyte rises, preferably to 3.8 or 4, while the pH of the anolyte drops appreciably below the pH of the liquid in the anodizing tank, perhaps to 0.5 or 0.6, depending on the rate of withdrawal of liquid from the anodizing tank and return of regenerated liquid to it.

Such change in pH is caused by conversion of aluminum dichromate into a more basic aluminum salt. The ultimate product is aluminum hydroxide, a gelatinous hydrogel, which may be formed according to the following equation:

Alz(OrzO1)s+ 61120 6 faradays (cathode compartment) 2A1(OH) BCIZO: 3H2

(cathode (anode (liberated) compartcompartment) ment) The catholyte may contain complex aluminum basic chromate compounds rather'than, or in ad'- dition to, aluminum hydroxide, but salts thus formed do precipitate and those which remain in solution are only mildly acidic.

Part of the catholyte may be drained from the cathode compartment of the regenerating tank to a separating or reservoir tank. Liquid containing aluminum hydroxide or hydroxyl chromate may be discarded continuously or periodically from this tank, or alternatively part of such liquid may be passed to a further regenerating tank where' it is subjected to an additional electrolytic re enerating action. By such a second electrolytic action it has been found possible to reduce the loss of chromic acid even further by converting a greater proportion of the aluminum dichromate to the hydroxide, or to a more basic hydroxyl chromate, before any portion of the liquid is discarded. While onl a single stage of electrolytic regeneration treatment accomplishes a very great saving in chromic acid, upwards of of the chromic acid radical still remaining in the liquid can be saved b passing it through. the second electrolvtic regeneration stage before the liquid containing precipitated aluminum salt is discharged from the system.

It will be evident that my regeneration process described may be, carried out in apparatus of different types. The current and voltage utilized in the re eneration cell may be varied accordin to the wishes of individual operators, and.

their selection will be governed by the ca acity and intensity of use 'of the anodizing tank, and

by the proportion of chromic acid recovered fromthe aluminum dichromate formed during the anodizing process. In the drawings, however, I have illustrated apparatus which I have found to operate effectively for the performance of my process. With very' little adjustment, by addition of a negligible amount of chromic anhydride, the pH of the anodizing electrolyte can be maintained substantially constant indefinitely, so that its periodic replacement is unnecessary.

The primary regeneration unit may include one or more cells within the tank I, three being illustrated in the drawings. Each cell includes a cathode l located centrally of a porous cup II, which preferably is of narrow rectangular shape to locate its sides quite close to the oathode. Each of these partition cups may be supported by a block l2 of acid-resisting ceramic material. A drain header l3 having a connection to the bottom of each cup ll leads to a separating tank 2 for removing the liquid from electrolytic influence. Anodes [4 are located intermediate adjacent cups II, and between the end cups and the end walls of tank I. These anodes and the cathodes I 0 are supported in any convenient fashion in such locations, and are connected to a suitable source of direct electric current 3.

The primary regeneration unit is supplied with electrolyte from the anodizing tank 4 by way of a pipe 40. The rate of flow through this pipe can be controlled by a valve 4|. Regenerated liquid is pumped from the regeneration tank I back to the anodizing tank by a pump 42 located in a conduit 43. The inflow end of this conduit is disposed slightly below the upper edge of tank I and establishes the level of the liquid in this tank. It may be swingable or otherwise adjustable to vary the height of its upper end, and consequ ntly the liquid level. The surface of the liquid in the anodizing tank 4 is somewhat above that in tank I, as shown, so that the liquid will flow by gravity through conduit 40 from the anodizing tank into the regenerating unit, and the discharge end of pipe 43 may empty into the top of the anodizing tank.

When electric current be ins to flow between the anode and cathode plates l4 and I0, respective y, of the regenerating unit the reaction represented by Equation 4 will occur, so that aluminum hydroxide or aluminum basic chromate will be formed within each of the porous cups I I. The liquid level in these cups should be maintained somewhat below the liquid level in the surrounding anode chamber by establishing the level of liquid in separating tank 2 slightlv below the desired level of liquid in cups II. The difference in hydraulic head between the anode and cathode compartments of tank I will cause the liquid to seep through the walls of cups H from the former to the latter as catholyte is withdrawn. The difierence in liquid level may be regulated either by raising the inflow end of pipe 43 or increasing the outflow of catholyte through pipe l3. In this way a continual flow of liquid is induced from the anode compartment into the cathode compartments, governed both by the hydraulic head and by the porosity of cup II.

The regeneration operation may terminate merely with removal from separating tank 2 of adulterated liquid containing precipitated aluminum salt through an outflow pipe 20 connected to the bottom of the tank. The discharge through this pipe may be controlled by swinging it to vary the height of its upper end for altering the difference in head between its outflow end and the level of the liquid in tank 2.

The gelatinuous precipitate of aluminum hydroxide, and perhaps aluminum basic chromates, formed during the regeneration operation, as previously mentioned, settle very slowly from suspension. I prefer, therefore, to electrolyze the solution in tank 2 again before rejecting the liquid containing such precipitate, thus to convert a greater proportion of the aluminum dichromate into chromic acid and aluminum hydroxide, or to change aluminum basic chromate into a less acidic and less soluble form. The extract from the regeneration system in such case is a slurry.

The liquid in tank '2 maybe recirculated back through cups II by a pump 2| withdrawing liquid from such tank through a pipe 22 and discharging it through small pipes 23 into the several cathode compartments. From this circuit a portion of the liquid may be withdrawn through a pipe 24 connected to the discharge side of the pump for flow into a secondary regeneration unit. The tank 5 of this unit is divided into two compartments by a porous wall 50, the anode 5| being located in one of these compartments and the cathode 52 in the other. Pipe 24 preferably discharges principally into the cathode compartment to enable a relatively high pH to be maintained in the anode compartment, which decreases resistance to current flow. In fact the pH in the anode compartment may be 0.6, acidic enough so that liquid from it could be returned directly to the anodizing tank 4. Little aluminum salt is present in the anode compartment, the principal electrolytic migration being of chromate radical from the catholyte through the partition 50 to the anolyte. Liquid may flow from the anode compartment either into the anodizing tank 4 by way of the return pipe 53, or, by gravity, back into tank 2. The anode 5i and cathode 52 are connected to a source of direct current, and may be connected to the same source 3 as that supplying the primary regenerating unit. When current flows through the electrolyte additional aluminum dichromate will be converted into aluminum hydroxide, according to the reaction of Equation 4, or complex aluminum hydroxyl chromate will be converted into more basic chromate or into aluminum hydroxide. This secondary regeneration operation, therefore, will serve to return additional chromate ions to the anode compartment, so that the catholyte will have even a higher pH than that of the catholyte in cups l, such as 4.4. The current flowing between the electrodes will be quite small because the conductivity of liquid having such a high pH as that in the cathode compartment is small.

As shown in Fig. 2 the tank 5 is quite long and the pipe 24 empties into one end of it. As the liquid passes along the tank toward the discharge end the pH of the anolyte will decrease while that of the catholyte will increase. The liquid adulterated with aluminum hydroxyl compound may be drained from the opposite end of the cathode compartment through a pipe 54. The rate of this flnal removal of liquid from the system may be governed by adjusting the height of the upper end of this pipe to vary the hydraulic head between it and the catholyte to control the flow through this pipe. Valve 25 in conduit 24 must be open at least sufliciently far to com pensate for the outflow through pipe 54, and should, of course, be adjusted relative to such flow so that a suflicient difference between the inflow through pipe 24 and the outflow through pipe 54 will be available to return chromic acid solution through pipe 53,remote from the inflow end of tank 5, to the anodizing tank or into tank 2.

It maybe found that during the regenerating operation the liquid in cathode cups l tends to become heated excessively because of the re-. sistance of the catholyte, particularly if it is not recirculated through tank 2, which serves as a reservoir, at a rapid rate. In order to overcome this condition the cathodes 10 may be in the form of tubes of copper or lead, through which coolin liquid is circulated. Thus a cooling liquid supply pipe I may be connected by nonconducting sections 16 to each of the hollow cathodes, while the liquid is returned from such cathodes through non-conducting tubes I1 into the return pipe 18. Any suitable source of cold water or other cooling liquid under pressure may supply the inflow pipe, and the rate of its flow through the electrodes can be governed by adjustment of valve I9 in this pipe.

The method of operating my equipment will be understood from the above description. The theory of the chemical reactions which occur during anodizing and regeneration of the anodizing electrolyte as set. forth in the above discussion affords the most logical explanation of the operation of my process which I have observed, but other or additional actions may occur. In any event, the catholyte liquid withdrawn from the system contains precipitate which is composed principally of aluminum, some chromium, and a small amount of other metals and silicon. I have been able to maintain the pH of the anodizing electrolyte substantially constant at a low and efiicient pH with the addition of a negligible amount of chromic acid over a long period of anodizing operation. Water will, of course, be added to replace the liquid drained from the system and lost during the anodizing and regenerating operation by evaporation.

I claim as my invention:

1. The continuous regeneration process which comprises electrolyzing partially spent aluminum anodizing chromic acid electrolyte in a regenerating unit which separates the anolyte and the catholyte by a porous partition, by such electrolyzing increasing the chromic acid content of the anolyte to decrease its pI-I, and forming aluminum hydroxyl compound in the catholyte, some as precipitate and some in solution, accompanied by an increase in the pH of the catholyte, continually withdrawing catholyte and precipitate contained therein, continually withdrawing anolyte for reuse as anodizing electrolyte, and continually supplying partially spent anodizing electrolyte to the regenerating unit to compensate for such withdrawal of catholyte and anolyte.

2. An aluminum anodizing process comprising anodizing aluminum in a chromic acid electroly-te, withdrawing electrolyte from the anodizing bath and delivering it to a regenerating unit which separates the anolyte and the catholyte by a porous partition, electrolyzing the anodizing electrolyte in such regenerating unit, by such electrolyzing increasing the pH of the catholyte to a pH of about 3.8 and decreasing the pH of the anolyte to a pH of about 0.6, returning anolyte having such a pH to the anodizing bath, and withdrawing catholyte having such pH.

3. An aluminum anodizing process comprising anodizing aluminum in a chromic acid electrolyte, continually withdrawing electrolyte from the anodizing bath and delivering it to a regen crating unit which separates the anolyte and the catholyte by a porous partition, electrolyzing the anodizing electrolyte in such regenerating unit, by such electrolyzing increasing the chromic acid content of the anolyte and forming aluminum 8 hydroxyl compound in the catholyte at least'aportionof' which is precipitated, continually returning anolyte to the anodizing bath and continua-lly withdrawing catholyte from the regenerating unit.

4. An aluminum anodizing process comprising anodizing aluminum in a chromic acid electrolyte, continually withdrawing electrolyte from the anodizing bath and delivering it to the anolyte of a regenerating unitwhich separates the anolyte and the catholyte by a porous partition, electrolyzing. the anodizing electrolyte in such regenerating unit, by such electrolyzing increasing the chromic acid content of the anolyte and forming aluminum hydroxyl compound in the catholyte at least a portion of which is precipitated, continually returning anolyte to the anodizing bath and continually withdrawing catholyte from the regenerating unit at a rate sufficiently rapid to maintain the liquid level of the catholyte substantially below the level of. the anolyte, thereby to induce seepage of anodizing liquid from the anolyte into the catholyte.

5. A method of regenerating aluminum anodizing chromic acid solution, comprising electrolyzing partially spent aluminum anodizing chromic acid solution while separating the anolyte and catholyte by a'porous partition to define anolyte and catholyte compartments, and thereby producing aluminum compound in the catholyte, Withdrawing such catholyte contain,- ing aluminum compound continually from the catholyte compartment, separating aluminum compound from such withdrawn catholyte, continually returning such catholyte freed substantially of aluminum compound to said catholyte compartment, withdrawing anolyte continually from said anolyte compartment, and substantially continuously supplying anolyte in the. form of partially spent aluminum anodizing solution to said anolyte compartment at the rate of. withdrawal of anolyte therefrom, whereby the withdrawn anolyte contains appreciably less aluminum than the supplied anolyte.

OSCAR R. BORNGESSER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 675,749 Merry et al June 4 1901 1,371,698 Linder Mar. 15, 1921 1,382,808 Sem June 28, 1921 1,408,618 McKee Mar. 7, 1922 1,986,920 Cross Jan. 8, 1935 1,990,582 Creighton Feb. 12, 1935 2,055,962 Boss Sept. 29, 1936 2,081,787 Boss May 25, 1937 2,085,002 Buzzard June 29, 193.7 2,099,658 Pearson et a1. Nov. 16, 1937 2,333,578 Knox, Jr., et al'. Nov. 2, 1943 FOREIGN PATENTS Number Country Date 22,819 Great Britain of 1892 OTHER REFERENCES The Journal of Industrial and Engineering Chemistry, vol. 12, No. 1, Jan. 1920, pp. 16-26.

"Plating-Polishing-Finishing, Feb. 1932, pp. 29-31. 

1. THE CONTINUOUS REGENERATION PROCESS WHICH COMPRISES ELECTROLYZING PARTIALLY SPENT ALUMINUM ANODIZING CHROMIC ACID ELECTROLYTE IN A REGENERATING UNIT WHICH SEPARATES THE ANOLYTE AND THE CATHOLYTE BY A POROUS PARTITION, BY SUCH ELECTROLYZING INCREASING THE CHROMIC ACID CONTENT OF THE ANOLYTE TO DECREASE ITS PH, AND FORMING ALUMINUM HYDROXYL COMPOUND IN THE CATOLYTE, SOME AS PRECIPITATE AND SOME IN SOLUTION, ACCOMPANIED BY AN INCREASE IN THE PH OF THE CATHOLYTE, CONTINUALLY WITHDRAWING CATHOLYTE AND PRECIPITATE CONTAINED THEREIN, CONTINUALLY WITHDRAWING ANOLYTE FOR REUSE AS ANODIZING ELECTROLYTE, AND CONTINUALLY SUPPLYING PARTIALLY SPENT ANODIZING ELECTROLYTE TO THE REGENERATING UNIT TO COMPENSTATE FOR SUCH WITHDRAWAL OF CATHOLYTE AND ANOLYTE. 